The disputed re-election of Mahmoud Ahmadinejad, the president, prompted a series of mass protests in Tehran after Mousavi and Mehdi Karroubi, another defeated candidate, claimed the June 12 vote was rigged.

But the demonstrations have largely died out in recent weeks after the authorities banned such gatherings and cracked down on protesters, political activists and journalists.

At least 20 peope were killed as the protesters clashed with security forces and pro-government Basij militia. More than 1,000 people were reportedly arrested.

Groups of students hold gatherings every year to remember the start of six days of student demonstrations on July 9, 1999.

The protests were sparked by the closure of the reformist Salam newspaper, but events turned violent after at least one student was killed during an attack on a student dormitory at the University of Tehran, by riot police and paramilitary forces.